1. Field
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved transmission scheme for a spread spectrum communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access techniques. A CDMA system provides certain advantages over other types of system, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “TIA/EIA/IS-856 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (5) some other standards. These standards are incorporated herein by reference. A system that implements the High Rate Packet Data specification of the cdma-2000 standard is referred to herein as a high data rate (HDR) system.
In a CDMA system, to achieve better utilization of the available resources, the cells within the system may be operated to transmit on a common frequency band (i.e., for a frequency reuse of one, or K=1). For this system, the transmission from each access point (i.e., base station) acts as interference to the transmissions from other access points. The interference is especially severe for a transmission received by an access terminal (i.e., a remote terminal) located at or near the boundary between cells. The interference may also be worse in a system designed to transmit at or near full power, if at all, such as the HDR system.
For a user-specific data transmission to a disadvantaged access terminal experiencing a high level of interference, the data rate of the transmission may be reduced to achieve the required signal quality for the desired level of performance (e.g., a one percent packet error rate). However, for an overhead channel, such as a control channel or a broadcast channel, designed to transmit messages to a number of access terminals within a cell, it is typically not practical to adjust the data rate on the overhead channel based on the operating conditions experienced by individual access terminals. Typically, a sufficiently low data rate is selected such that the overhead channel may be properly received by a large percentage of the access terminals within the cell. For example, the control channel in the HDR system is transmitted at a low data rate of 38.4 kbps or 76.8 kbps, while a traffic channel may be transmitted at data rates ranging from 38.4 kbps to 2.4576 Mbps depending on the link conditions.
In certain instances, a transmission at even the lowest system data rate (e.g., 38.4 kbps) may not be properly received by a disadvantaged access terminal because of excessive interference from other cells. In certain other instances, it may be advantageous to transmit the overhead channel at higher data rates (e.g., to provide increased transmission capacity on the overhead channel, to reduce the delay or latency of control messages transmitted on the overhead channel, to allow data transmission on traffic channels to start sooner if it is dependent on a control message, and so on).
As can be seen, a transmission scheme that may be used to reduce interference and which may support a particular desired data rate (e.g., the lowest system data rate or a higher data rate) in a CDMA communication system is highly desirable.